The present invention relates to a modulator-demodulator (modem) device for a phase modem system, a quadrature amplitude modem system, and the like. The present invention relates particularly to a device capable of detecting an unsynchronized frame state in synchronization transmission in a modem device. The device according to the present invention is used, for example, for a modem in terminal devices for data communication.
In a synchronization type data transmission system adopting a phase modem system, a quadrature amplitude modem system, and the like, frame synchronization between the transmitter side and the receiver side is established by introducing a frame synchronization signal in a training signal for the automatic pulling-in of the automatic equalizer of the receiver side prior to the transmission of the data. Data transmission is carried out with the synchronized frame phase which is established when receiving the training signal. If the frame phase deviates due to a variation in the characteristics of the data transmitting network, and an unsynchronized frame state occurs, normal data transmission is prevented. However, this unsynchronized frame state can be detected only by an abnormal condition of received data. Thus, it would be desirable if a system in which the frame synchronization is detected quickly and reliably could be realized.
In general, for a transmission device which uses an analog line having a transmission rate of 2400 bps to 19200 bps, data transmission speed is normally set to an integer number times 2400 bps. Thus, in a transmission device in which the modulation speed has an interval for synchronizing data and for transmitting and receiving synchronized data, which is different from 2400 bps, and the number of bits which can be transmitted at one time is an integer, it is necessary to carry out transmission processing in a frame synchronized condition between a transmitter and a receiver by using frame phase information in order to coincide with the transmission speed of an integer multiple of the standard 2400 bps.
In a prior art system, the frame phase synchronization between a transmitter side and a receiver side is established by introducing a frame synchronization signal in a training signal prior to data transmission in order to make the frame phase of the receiver side coincide with the frame phase of the transmitter side, and during the data transmission, the operation is carried out with synchronized frame phase when the training signal is received.
For example, if one frame is assumed to be constituted by 8 modulations, a modulation synchronization signal is derived by demodulating each instance of modulation, frame phase information is generated by counting the number of these demodulated synchronization signals, and the timing of clearing a counter is preliminarily determined by the frame phase synchronization signal when the training signal is received. Frame phase synchronization between the transmitter side and the receiver side can be established even if the frame synchronization signal is not received during data transmission.
However, in such a prior art frame synchronization system, if part of the modulation signal within one frame is lost due to a variation in the characteristics of the transmitter, such as distortion of the signal waveform caused by noise during the data transmission, a momentary loss of signal caused by a cut of the line, or the like, the frame phase on the receiver side deviates and an unsynchronized frame state occurs. Once the unsynchronized frame state occurs, frame synchronization cannot be recovered, normal data transmission is prevented, and this unsynchronized frame state can only be detected by the abnormal state of the received data, which causes a problem in operation.